The present invention relates to a barrier screw having improved solids conveying capability. The barrier screw is used in a plasticizing device for converting solid thermoplastic material to molten material.
In the preparation of plastic materials utilizing a single screw extruder and a conventional single stage screw having feed, transition and metering sections, the raw or unmelted thermoplastic is introduced into the feed section and advanced to the end of the metering section by means of a helical flight and rotation of the screw. As the thermoplastic material travels along the screw, it is heated by means of heat conduction from the heated barrel and by shear between the metal surfaces of the rotating screw, the stationary plasticizing barrel, and the plastic itself. It has been found that some unplasticized pellets become encapsulated in the hotter material or melt and travel the length of the screw without being fully melted and attaining uniformity of texture. Accordingly, improvement in the quality and uniformity of the melt from a plasticizing device has been a continuing source of focus by workers in this art.
A class of devices known as barrier screws have been developed in an attempt to deal with these problems. These devices typically have a plasticizing or transition zone with two parallel channels. A secondary or barrier flight is started usually at the beginning of the transition zone and creates the two channels, namely a solids channel and a melt channel. The barrier flight is undercut below the primary flight allowing melted plastic to pass over it. Typical barrier screws are illustrated in the article "High-yield Extruder Screw Melts and Pumps In Step," by Heung-Tai Kim, Plastics Engineering, August, 1985, pp. 27-30, in Japanese Patent Publication Nos. 56-92039 and 57-84830, and in U.S. Pat. No. 4,330,214 to Willert.
The Willert barrier screw has a feed zone, a plasticizing zone and a metering zone. The screw in the feed zone has a single primary flight which forms a single feed channel for moving solid material to the plasticizing zone. At the beginning of the plasticizing zone, a barrier flight is introduced to divide the feed channel into a solids channel and a melt channel. The solids channel is initially deeper than the feed channel but gradually decreases in depth as it approaches the end of the plasticizing zone. The melt channel on the other hand is shallow at the beginning but becomes deeper as it approaches the end of the plasticizing zone. As the melt channel deepens, the thickness of the shank decreases. One of the problems associated with this screw is the tendency for the shank to break in these thin regions.